1. Field of the Invention
The present invention relates to a method for manufacturing a grain oriented electrical steel sheet for use as iron cores of electric apparatuses such as transformers and the like. More specifically, the present invention relates to a method for manufacturing a high magnetic flux density grain oriented electrical steel sheet, in which inhibitors for restraining the growth of primary recrystallization grains are formed after a cold rolling is carried out to the final thickness, thereby making it possible to carry out a low temperature heating.
2. Description of the Prior Art
The grain oriented electrical steel sheet has a (110) [001] texture in the rolling direction. The method for this was first disclosed by N. P. Goss, and since that time, many researchers have made efforts to improve the method and the properties of the steel sheet. The magnetic properties of the grain oriented electrical steel sheet appear in the secondary recrystallization structure which is obtained by inhibiting the growth of the primary recrystallization grains and by selectively growing the (110) [001] crystal grains from among the inhibited crystal grains.
Therefore, if the grain oriented electrical steel sheet having superior magnetic properties is to be manufactured, it is important how the inhibitors are formed, and how the processes of obtaining a stable (110) [001] texture from among the inhibited grains are constituted.
Specifically, the inhibitors are formed by employing fine precipitates and segregated elements. The precipitates should be uniformly distributed in a sufficient amount and proper sizes, so that the growth of the primary recrystallization grains can be inhibited until the formation of the secondary recrystallization grains. Further, the precipitates should not be decomposed by being maintained in a thermally stable state up to the peak temperature immediately before the formation of the secondary recrystallization grains. The currently used inhibitors which satisfy the above conditions are MnS, MnS+AlN, MnS(Se)+Sb.
The technique of manufacturing the electrical steel sheet by using only MnS is disclosed in Japanese Patent Gazette Sho-40-15644. In this technique, a stable secondary recrystallization structure is obtained by carrying out two stages of cold rolling including an intermediate annealing. However, this method cannot obtain a high magnetic flux density, and the manufacturing cost is increased due to the fact that the two stages of cold rolling are carried out.
The typical technique of manufacturing the oriented electrical steel sheet by using MnS+AlN as the inhibitors are described in Japanese Patent Gazette Sho-30-3651. In this method, a single stage of cold rolling is carried out at a reduction rate of 80% or more, thereby obtaining a high magnetic flux density. However, if this method is applied to the industrial field, the manufacturing conditions are too stern, and therefore, the respective process conditions have to be strictly controlled.
Specifically, in this method, a high temperature slab heating, a hot rolling, a precipitation annealing, a cold rolling, a decarburization annealing and a high temperature annealing are carried out.
Here, the high temperature annealing refers to the process of developing the (110)[001] texture by making the secondary recrystallization occur in the final gauged sheet In any method using the inhibitor, an annealing separator is spread on the steel sheet before carrying out the high temperature annealing to prevent the sticking of the sheets, and during the decarburization, the oxide layer of the surface of the steel sheet reacts with the annealing separator to form a glass film, thereby providing an insulating property on the steel sheet. Thus, by the high temperature annealing, the final product of the steel sheet having the (110)[001] texture is provided with an insulating film on its surface.
The typical technique of manufacturing the grain oriented steel sheet by using MnS(Se)+Sb as the inhibitors are disclosed in Japanese Patent Gazette Sho-51-13469. In this method, a high temperature slab heating, a hot rolling, a precipitation annealing, a first cold rolling, an intermediate annealing, a second cold rolling, a decarburization annealing and a high temperature annealing are carried out. In this method, a high magnetic flux density can be obtained. However, two stages of cold rolling are carried out, and Sb or Se which is very expensive is used as the inhibitor. Therefore, the manufacturing cost is increased, and, still more, the production line shows to be toxic to the human body.
Further, in the above methods, the steel slab is heated at a high temperature for a long time to realize solid solutions of MnS or AlN before carrying out the hot rolling. Then during the cooling of the hot rolled sheet, MnS or AlN is formed into precipitates of proper size and distribution, thereby making it possible to use them as the inhibitor.
Specifically, in order to achieve a high magnetic flux density, it is known that a slab-heating has to be carried out up to 1300xc2x0 C. in the method using MnS as the inhibitor, a slab-heating has to be carried out up to 1350xc2x0 C. in the method using MnS and AlN as the inhibitor, and a slab-heating has to be carried out up to 1320xc2x0 C. in the method using MnS(Se)+Sb as the inhibitor. Actually when it is applied to the industrial production, the heating has to be carried out up to 1400xc2x0 C. to obtain a uniform temperature up to the inner regions of the slab.
In the case where the slab is heated to a high temperature for a long time, the consumed heat amount is large, and therefore, the manufacturing cost is increased. Further, the surface portions of the slab are melted down, with the result that the repair cost for the furnace is increased, and that the life expectancy of the furnace is shortened.
Particularly, if the columnar crystal (the solidified structure) of the slab surface is coarsely developed, then deep lateral cracks are formed during the later hot rolling. As a result, the yield is markedly decreased, and other problems may occur.
In order to solve the above described problems, if the slab-heating temperature is lowered when manufacturing the grain oriented steel sheet, then many advantages can be obtained in the manufacturing cost and the yield.
Therefore recently, research has been briskly carried out on the methods in which MnS requiring a high solid solution temperature is not used. That is, in these methods, the precipitates as the inhibitors are not formed by only the elements added in the steel making process, but the precipitates are formed at a proper stage during the manufacturing process.
The above methods are described in Japanese Patent Gazette Hei-1-230721 and Hei-1-283324 in which nitrogenization treatment is applied.
Belonging to this category, the following can be cited. One is that in which an annealing separator containing a chemical agent capable of nitrogenization is spread on the steel sheet to nitrogenize the steel sheet. Another is that in which a gas capable of nitrogenization is put into the atmospheric gas during the heating stage of the high temperature annealing to nitrogenize the steel sheet. Still another is that in which the steel sheet is nitrogenized within an atmosphere capable of nitrogenization after the decarburization.
Japanese Patent Gazette Hei-2-228425 discloses a method in which precipitates are formed by putting nitrogen into the steel during a nitrogenization process carried out on the hot rolled steel sheet, or on the first cold rolled steel sheet.
Japanese Patent Gazette Hei-2-294428 discloses a method in which nitrogenization and decarburization are simultaneously carried out during a decarburization annealing after the cold rolling. In this method, (Al,Si)N is used as the inhibitor, and due to the nitrogenization which occurs simultaneously with the decarburization, (Al,Si)N are formed mainly on the grain boundaries of the surface layer, so that the growth of the primary recrystallization grains of the surface layer can be inhibited. Accordingly, the surface layers have fine primary recrystallization grains, while the internal regions have coarse recrystallization grains. As a result, the secondary recrystallization becomes unstable, and consequently, the magnetic flux density is lowered.
In an attempt to solve this problem, Japanese Patent Gazette Hei-3-2324 discloses a method in which first the decarburization annealing is carried out, and after the growth of the grains to a certain size (about 15 xcexcm), a nitrogenization is carried out by using ammonia gas during an additional decarburization annealing.
In these methods, the nitrogen which is produced during the decomposition of ammonia at above 500xc2x0 C. is put on the steel sheet.
The nitrogen which has intruded into the steel sheet reacts with the surrounding Al and Si to form nitrides, and these nitrides are utilized as the inhibitor. The inhibitors in this case are mainly Al nitrides such as AlN and (Al,Si)N.
As described above, the methods in which the low temperature slab heating is carried out utilize the contained chemical agents capable of nitrogenization or the gas capable of nitrogenization, thereby realizing the nitrogenization. Thus precipitates are formed within the steel sheet so as to manufacture the grain oriented electrical steel sheet.
However, in all the methods, the steel sheet commonly contains about 0.050% of carbon, and thus the nitrogen can be put to the steel sheet after a decarburization. As a result, the additional subprocess becomes necessary. Particularly, in the method using the gas for nitrogenization, a new facility or a drastic modification of the existing facility has to be added. Further, in the method adding chemicals capable of nitrogenization to the annealing separator, large amounts of defects are generated in the surface forsterite layer.
Further, the amount of S or N within the steel is relatively high, and therefore, an unintended MnS or AlN is produced in large amounts after the hot rolling. After the decarburization, it causes the size of the primary recrystallization grains to be fine, and therefore, in order to achieve a stable secondary recrystallization, a very strong inhibitor is to be prepared. That is, fine precipitates have to be formed with uniform distribution. For this purpose, the sizes of the grains have to be controlled to a small range in a stern manner after the decarburization, and the amount of the nitrogenization has to be strictly controlled. Therefore, the industrial application becomes very difficult.
If the nitrogenization method is to be applied to the industrial field, the following two problems have to be solved primarily.
First, the process has to be improved without much modifying the existing facility. This is the economic aspect of the new method.
Second, a stable grain oriented electrical steel sheet should be able to be manufactured with a wide tolerance for the process control. This is related to the yield, and ultimately to the manufacturing cost.
In order to solve the above described problems of the conventional techniques, the present inventors carried out studies and research, and based on the results, the present inventors came to propose the present invention.
Therefore it is an object of the present invention to provide a method for manufacturing a grain oriented electrical steel sheet, in which a silicon steel slab having a lowered C content and a proper B content is reduced to the final thickness, and a nitrogenization is carried out at proper conditions to form BN precipitates, so that a low temperature heating for the slab will be possible, that the electrical steel sheet can be manufactured without modifying the existing facility, and that a uniform primary recrystallization structure can be obtained after the nitrogenization, thereby obtaining a high magnetic flux density.
It is another object of the present invention to provide a method for manufacturing a grain oriented electrical steel sheet, in which a silicon steel slab containing a lowered C amount and proper amounts of Cu, Cr and Ni is reduced to the final thickness, and nitrogenization is carried out at proper conditions to obtain a uniform primary recrystallization structure, so that a low temperature slab heating will be possible, and that the electrical steel sheet can be manufactured without modifying the existing facility, thereby obtaining a high magnetic flux density.